Lens feed system



Sept. 13, 1966 R. L. TANNER 3,273,154

LENS FEED SYSTEM Filed May 27, 1964 2 Sheets -Sheet 1 4615 E2 -jkx INVENTOR. v ROBE/e7 L. T4 AWE 2 BY J a 2 2 A TTORNEY 2 s n T I N E N X E 4 Ek L DP W E O M c: A. 4 D. A. M\| Am RT 1 I- i l Hdi l l l M Z A 3 I 2 I M B 1111 X O 6 R 6 2 UOH 2 1| G M M L 5 5 2 United States Patent 3,273,154 LENS FEED SYSTEM Robert L. Tanner, Menlo Park, Calif., assignor, by mesne assignments, to Control Data Corporation, South Mmneapolis, Minn, a corporation of Minnesota Filed May 27, 1964, Ser. No. 370,471 9 Claims. (Cl. 343-753) This invention relates to antennas of the type known as wire grid lens antennas, and more particularly to an improved arrangement for exciting said antennas.

In Patent No. 3,234,556 for a Broadband Biconical Wire-Grid Lens Antenna Comprising a Central Beam Shaping Portion, and Patent No. 3,234,557 for a Non- Uniform Wire Grid Len-s Antenna, both by this applicant, there is described and shown a wire grid lens antenna of a type with which the present invention finds its optimal use. These wire grid lens antennae may be broadly described as comprising a central lens region surrounded by a horn region. Both the lens and the horn are'made of wire grid mesh. A central lens region comprises a pair of spaced overlying conductive wire grids substantially in the form of two circular concave discs, with their convex surfaces opposing each other. The horn region flares outwardly from the disc peripheries. Such an antenna has excellent broad band characteristics for operation over bands lying within the frequency range extending from below one to above 1,000 megacycles per second.

The problem which is presented with antennas of this type is the one of applying excitation, or feeding the antenna, in a manner which is most efficient and which does not introduce undesirable side radiation eifects.

An object of this invention is the provision of a novel arrangement for feeding a wire grid lens antenna of the general type described.

Another object of the present invention is the provision of a useful arrangement for applying excitation to a wire grid lens antenna which has excellent coupling etficiency.

Still another object of the present invention is the provision of a feed arrangement for a wire grid lens antenna which substantially eliminates undesired side radiation effects due to said feed.

These and other objects of the present invention may be achieved by a feed structure which is installed at spaced points around the edge of the lens portion of the structure and extends from the junction between the lens and the horn for a predetermined distance inward toward the center of the lens. The feed structure substantially comprises a transmission line which is connected to the transmitter by an input network at the lower grid element of the lens and extends diagonally upward to couple to the upper lens grid element through an output network at a predetermined distance from the edge of the lens. This feed is hereafter called a ramp feed. The ramp feed can produce undesirable side radiation. In accordance with this invention dipole elements are provided and are excited in phase quadrature with the ramp element to thereby eliminate said undesirable side radiation.

The novel features that are considered characteristic of this invention are set forth with particularity in the appended claims. The invention itself both as to its organization and method of operation, as well as additional objects and advantages thereof, will best be understood from the following description when read in connection with the accompanying drawings, in which:

FIGURE 1 is a perspective view of a wire grid lens of the general type with which this invention is concerned.

FIGURE 2 is a schematic diagram of a ramp feed element in accordance with this invention, for a wire grid lens.

FIGURE 3 is a hypothetical illustration of a segmented ramp shown to assist in an understanding of this invention.

FIGURE 4 is a hypothetical illustration of an end-fire array which is equivalent to the segmented ramp shown in FIGURE 3.

FIGURE 5 is a diagram in elevation illustrating the appearance of a ramp element with capacitive loading, in accordance with this invention.

FIGURE 6 is an idealized diagram of a ramp feed with end elements.

FIGURE 7 is a circuit diagram of the preferred coupling of the ramp feed and dipole end elements to the generator and to the lens element, in accordance with this invention.

FIGURE 1 is a perspective view of a wire grid lens antenna of the general type which has been described and claimed in the previously-indicated patent applications by this inventor. The antenna has a center portion 10 which forms a wire grid lens for azimuthal beam shaping, and a peripheral portion 12 which is the horn or radiating structure for elevational beam shaping and for matching the impedance between wire grid lens '10 and the surrounding space. The radiation structure 12 is shaped in the form of a biconical or radially flared horn.

The wire grid lens 10 includes an upper wire grid 14 and a lower wire grid '16 spaced in opposite and overlying relationship by means of a plurality of nonconductive suspensionor support members 18. As a practical matter, the peripheral portion of the upper wire grid 14 and the lower wire grid 16 may be secured through rim members, such as a pair of metal rings 20, 22, respectively, which are light in weight and which form con venient conductive terminating support edges. Furthermore, these rings also provide a convenient means for attaching the radiating structure 12 thereto. The lens wire grids 14 and 16 are formed of composite metallic wires in the form of a mesh structure. Thus, the upper and lower lens elements are substantially in the form of spaced nesting saucers.

The radiating horn structure 12 has an upper and a lower portion respectively 24, 26, also extending outwardly to support rings respectively 28,30, which are attached to suitable guide pole supports 32. These pole supports are spaced around the periphery of the biconical horn structure and assist in maintaining it spread out with the proper desired horn shape.

By way of illustration, and not by way of limitation, in the frequency region with which this antenna is employed, the lens portion of the antenna may have a diameter of 600 feet and thedistance of separation between the upper and lower elements of the lens at the center is on the order of 6 inches and at the other periphery is onthe order of 12 feet. The biconical horn portion extends outwardly still further from the periphery of the lens portion. The feeds in accordance with this invention, are installed at the edge of the lens portion and may extend from the junction between the lens and the horn portion for a distance on the order of one-fourth of the lens radius toward the center of the lens. In this region, the spacing between the grids of the lens is large enough so that the lens elements present approximately parallel conducting planes. In an embodiment of the invention which was built and operated satisfactorily, thirty-six feeds were employed, spaced around the lens.

As will become clear from a reading hereof, a preferred embodiment of this invention comprises the structure shown in FIGURES 6 and 7. The description preceding that of FIGURES 6 and 7 is directed to the theory and reasoning leading to the structures shown therein. It is provided in order to assist in an understanding of this invention.

FIGURE 2 is a schematic diagram of a ramp feed element, for a wire grid lens, and the representation of the grid lens is indicated here as comprising parallel conducting planes. At the outer edge of the lower grid element 16 there are terminalsAO, to which excitation for the grid lens antenna is applied. In accordance with this invention, the principal component of feed, known as a ramp 42, extends between the terminals 40 toward the upper lens element 14 and connects thereto through a terminating resistor 44. The ramp feed element may comprise a cylindrical conductor.

Although there are several ways of viewing the coupling between the ramp element and the Wave modes in the lens, the simplest of these regards the ramp as a traveling wave line source of current elements. It is evident that the ramp element between the two conductive planes of the grid is a type of transmission line.

FIGURE 3 is an illustration of a hypothetical segmented ramp shown to assist in an understanding of this invention. If it is assumed that the ramp element is fed by a generator 46, which is connected between the terminals 40, a current wave will flow up the ramp toward the right, as represented by the arrow. Hypothesizing further, for simplicity, that the wave is unattenuated and that no reflection occurs from the end, the current at all points to the right of the terminals 40 will be retarded in phase relative to the input current by an amount proportional to the distance from these terminals. This condition can be expressed mathematically by the equation where I (x) is the ramp current at a distance x from the terminals and I is the current at the input. The factor k(=21r/)\) is the propagation constant for the wave on the ramp.

As the current wave travels along the ramp, it couples to waves between the grids. The reason for this coupling is that the waves existing between the grids have their electric field lines generally vertical, and the current on the ramp has a vertical component. General electromagnetic coupling theory tells us that the excitation of a wave mode by a current element is proportional to the component of current lying along the direction of the electric field of the mode. Thus, only the vertical component of the ramp current is effective in exciting the wave modes in the lens.

Although the coupling to these modes is actually continuous over the length of the ramp, it is convenient in helping to visualize it, and also for purposes of making numerical computations, if it is assumed that the current acts at discrete points. As an example, consider again FIGURE 3. Here, the ramp is considered as broken into five segments. Each segment can then be replaced approximately by an entirely vertical current element situated at the center of the segment and having a phase corresponding to the phase of the ramp current at that point. The vertical current elements could have lengths equal to one-fifth the spacing between grids, and carry currents equal to the ramp current, or they could extend the entire distance between grids but carry currents only one-fifth the ramp current. This latter representation is illustrated in FIGURE 4. (In this example, five segments are used for illustration only. In actual computations, it is normally necessary to assume a larger number of segments to obtain the desired degree of accuracy.) In FIGURE 4, each one of the indicated current segments is supplied by each of the five current generators, 46A through 46E.

When the ramp is replaced by the array of current elements shown in FIGURE 4, its radiation properties become more obvious. An array of the, form shown,

with the current phases as given, is readily recognized as an end-fire array which launches a beam directed toward the right. In this case, the radiated energy is confined by the grids to the space between them.

In the preceding discussion, a number of idealizations are made for simplicity. One of these, of course, is the assumption that the current wave is unattenuated. In the actual situation, radiation from the rampthat is, energy being coupled out of the transmission line mode on the ramp and into the propagating wave mode in the lenscauses the current wave on the ramp to be quite strongly attenuate-d. This face does not alter the basic picture as presented, however. 7

Another idealization is that the ramp, as shown in FIGURE 2, is a uniform cylinder. Viewed as a transmission line, such a structure would have a characteristic impedance which varied greatly over the length of the ramp. At the input end, where the ramp is close to the lower grid, the impedance would be low. In the center, where the spacing to both grids is large, the impedance would be high, but would become low again as the ramp approaches the upper grid. To avoid such variation in characteristic impedance and the mismatch and other undesirable electrical effects which would result, the constant diameter cylinder shown in FIGURE 2 must be replaced by a conductor whose effective diameter tapers from a small value at the ends of the ramp near the grid planes to a large value near the center.

FIGURE 5 is a view, in elevation, of the preferred arrangement for the ramp feed. The details of the preferred structure used to couple this ramp feed to the generator at one end and to the upper lens element at the other end are shown in FIGURE 7. The required taper in effective size is obtained by using two conductors, 50, 52 (which are, in fact, wires of the same material composing the grids) whose spacing varies as a function of distance along the ramp. It is well known, of course, that the electrical size of a composite conductor composed of two wires depends upon both the size of the individual wires and the spacing between them. When the composite conductor is substantially separated from. other parts of the system as, for example, in the case of the legs of a rhombic antenna, its effective radius is given by the geometric mean of the radius of the individual wires and the spacing between them. In the geometry imposed by the lens, where the composite conductor is enclosed in the relatively narrow space between the two grids, the relationship is very much more complicated, but can nevertheless be analyzed.

A further deviation between actual conditions and the idealized situation discussed is due to the fact that the wave modes in the grid propagate at somewhat less than free space velocity. For proper coupling to the lens wave modes and to achieve a more nearly optimum. feed radiation pattern, the wave on the feed should propagate at a velocity slightly lower than the waves in the lens. This means that the wave on the feed should travel substantially below free space velocity. In the actual feeds, the required wave slowing is accomplished by capacitive loading on the ramp. In accordance with this invention, the required capacitive loading is provided by formed sheet metal pieces 54A through 54H connected between the wires forming the ramp and supported by the impregnated wood struts 56A through 56H which support the ramp wires.

While the ramp just described is the principal coupling element of the feed, used by itself it has certain deficiencies. These result from the fact that the ramp can be considered as an end-fire array of electric dipoles. The basic element pattern of an electric dipole is circular, or omniazimuthal. When such elements are arranged in an end-fire array, their fields add in the direction of the array and tend to cancel in the direction toward the rear. In the directions at right angles to main beam direction, however, no cancellation occurs and the radiated field tends to be quite high, particularly at frequencies where the array is relatively short electrically. As an example, consider a uniformly illuminated end-fire array one-half wavelength long. The radiation pattern of such an array is a cardioid in which radiation toward the rear is zero, but radiation at right angles to the array is only 3.5 db below that in the principal direction.

When used in the lens, a feed having such high side radiation tends to excite higher order circumferential modes in the lens. These in turn give rise to undesirable, high side lobes and back lobes in the lens radiation pattern. In order to cancel out the side radiation from the feeds, end elements are employed. FIGURE 6 is an idealized illustration of a ramp 58 .plus end elements 60, 62 positioned adjacent either end. If the reference current, I be the current at the input to the ramp and it is required that the currents in the end respective elements have the respective magnitudes and phases respectively I =I e" For such an arrangement, it can be shown that the effect of the end elements is to exactly cancel side radiation from the ramp at all frequencies without altering radiation in the principal direction.

A few comments are in order concerning the current relations shown in FIGURE 6. In particular, note the relationship 1 ok/znA j ov/zmA Ov/wA Where I is the current at the input end of the ramp. I is the current in the dipole element at the input end. A is the total length of the ramp, and A is the wavelength of the wave on the ramp. In the second expression of the relation, v is the velocity of the Wave on the ramp and f is the frequency. It will be observed that the factor 1' in the expression means that the dipole current must lead the ramp current in phase by 90 degrees. On the other hand, the factor occurring in the numerator of the expression means that the current in the dipole must fall olf inversely with frequency. Thus, at a frequency where the ramp is one-half wavelength long,

and

In other words, the dipole current is about 30 percent of the ramp current. If the frequency is increased to a value twice as great, A=)\, and |I |=l/21r|I The dipole curent is about 15 percent of the current in the ramp.

,Because it is designed to look as nearly as possible like a constant impedance transmission line, the ramp presents to a generator an impedance which is very nearly a constant resistance, so that if a voltage source is connected to it, a current will flow which is in phase with the volt-age. This relation is expressed by the equation I V/R l V/jwL (4) Substituting into Equation 4 an expression for the voltage V obtained from Equation 3, and recognizing that From Equation 5 it may be seen that in an arrangement of the type described, in which the voltage source is applied to the parallel connection of the ramp and an end dipole element designed to have the correct value of inductance, the current in the dipole has the correct frequency dependence relative to the current in the ramp. However, the current in the dipole, instead of leading the ramp current by 90 degrees, lags it by 90 degrees. 7 In other words, it is 180 degrees out of proper phase. This deficiency can be remedied by means of a phase inverting transformer.

While the insertion of a phase inverting transformer between generator and ramp would provide the proper current relationships at the input to the ramp, it has certain deficiencies, however. First, since the impedance it presents to the generator is essentially that of an inductance in parallel with a resistance, it can never look like a perfectly matched load. Because the current flowing in the dipole is relatively small compared with the current in the ramp, the mismatch will not be great, although at low frequencies where the dipole current is greatest, it will be significant. A second difiiculty is the fact that the dipole impedance contains a resistive component due to radiation. This precludes obtaining the correct -degree phase relationship. These problems are both overcome by employing a circuit arrangement commonly known as a constant resistance network. This is a circuit having two parallel connected branches. One branch contains a resistance in series with a capacitance and the other a resistance in series with an inductance. If

these resistances are equal and have the value R= /L/ C, then the impedance of the circuit is a constant resistance equal to R. Moreover, the current in the inductive branch has the desired inverse frequency dependence relative to the current in the capacitive and lags it by exactly 90 degrees at all frequencies.

It will also be noted that, in this circuit, only the ratio L/ C need be fixed to make the circuit a constant resistance network. The ratio of the current in the two branches at any given frequency is determined by the product of L and C.

Reference is now made to FIGURE 7 which is a circuit diagram showing the preferred ramp feed input and output net-work, in accordance with this invention. One output terminal of the generator 46 is connected to one end of the primary and secondary windings respectively 64A, 64B of the phase reversing transformer 64, and also to the lower lens element 16. The other output terminal of the generator is connected to the other end of the primary winding 64A through a resistor 66, and is also connected through a capacitor 68 to the ramp wires 50, 52. The other end of the secondary winding 64B is connected in series with an inductance 70, which in turn is connected in series with a resistance 72 connected to dipole element 74. The other end of the dipole element is connected to the upper grid 14.

The transformer 64 may be a specially wound, ferrite core, having a unity turns ratio. The resistance of the inductive branch of the constant resistance network is provided by a combination of radiation resistance of the dipole element, the winding resistance of the transformer and some resistance added in the primary side of the transformer. The inductance of the inductive branch of the constant resistance network is comprised of the dipole inductance, the leakage inductance of the transformer, and some additional trimming inductance 70. The resistance of the capacitive branch is provided, of course, by the ramp. Thus, the circuit arrangement shOWn in FIGURE 7 provides the required current magnitude and phase relationships and still presents to the generator an impedance which essentially is a pure resistance.

A problem rather similar to that at the input end arises in feeding the dipole at the termination end of the ramp. Here, the current source is the ramp itself, since it provides the most convenient method of bringing power from the generator to the termination dipole. When there is taken into account the phase reversal which results from feeding the end dipole at the top rather than at the bottom, as in the case of the input dipole, the problem again exists of exciting a downward directed current in the dipole which leads the current in the ramp by 90 degrees.

In addition, there is the requirement to present to the ramp, at its termination on the upper lens element, a

matched impedance. At the upper end use is also made of the constant resistance network principle, a phase inverting transformer, and the inductance of the dipole element. Thus, the ramp wires 50, 52 connect to the junction between a resistor 76 and a capacitor 78. The resistor 76 connects to one end of the primary winding 80A of the transformer 80. The other end of the primary winding connects to the upper lens element. A resistor 82 connects capacitor 78 to the upper grid element 14. The transformer secondary winding 80B has one end connected to the upper lens element 14 and the other end to a trimming inductance 84. A dipole element 86 is connected between the lower grid element and the inductance 84.

If the circuit is analyzed in detail, it is observed that it is not possible to make the downward current in the dipole lead the ramp current by exactly 90 degrees. This phase relationship can be approached quite closely, however. In addition, since the current at the termination of the ramp is substantially attenuated by radiation, the current in the terminating dipole is small relative to the current in the input dipole, and a slight phase discrepancy 4 can be accommodated with less adverse effect on the pattern.

There has accordingly been described and shown here in a novel, useful and eflicient feed arrangement for a wire grid lens antenna.

What is claimed is:

1. In a wire grid lens antenna of the type wherein said wire grid lens comprises an upper and a lower lens element each being made of wire grid and each having substantially the shape of a disc, both resembling two spaced nesting shallow saucers, said upper and lower elements being spaced a predetermined distance from one another, an improved feeding structure comprising conductor means extending from an edge of one of said two spaced lens elements radially inwardly to the surface of the other of said two lens elements, means for applying excitation between said edge and said conductor means, and means for coupling said conductor means to the surface of said lens element to which it extends.

2. The structure recited in claim 1 wherein said conductorsmeans includes a first and a second conductor, and means for varying the spacing between said first and second conductor along the length thereof to produce a characteristic impedance which is substantially constant over the entire length of said conductor means.

3. The structure as recited in claim 2 wherein there is included means for capacitively loading said conductor means for reducing the velocity of wave propagation along said conductor means below that of wave propagation in said lens.

4. In a wire grid lens antenna of the type wherein said wire grid lens is made of two circular wire mesh discs spaced from one another the improvement in the feed to said lens antenna comprising conductor means extending from an edge of one of said two wire mesh discs radially inwardly to the surface of the other of said wire mesh discs a predetermined distance, means for applying excitation to the one end of said conductor means which is near the edge of said one of said discs, said means for applying excitation including a first constant resistance network means, and a first radiating means for substantially eliminating side radiation from said conductor means, and means coupling said other end of said conductor means to said other wire mesh disc surface including a second constant resistance network means, and a second radiating means for canceling undesirable radiation from said conductor means.

5. In a wire grid lens antenna of the type wherein the wire, grid lens has the form of two spaced nesting saucers, and there is a feed system for said antenna including conductor means extending radially inwardly from an edge of one of said saucers a predetermined distance to the surface o f the other of said saucers, the improvement comprising means for minimizing unwanted radiation from said conductor means comprising a first and a second radiating element respectively positioned at the two ends of said conductor means and extending between said two saucers, and means for exciting each of said radiating elements with currents phased in quadrature with the exciting current in said conductor means.

6. In a wire grid lens antenna of the type wherein the wire grid lens has the form of two spaced nesting saucers, and there is a feed system for said antenna including conductor means extending radially inwardly from an edge of one of said saucers a predetermined distance to the surface of the other of said saucers, the improvement comprising a transformer having a primary and a secondary winding, means connecting said transformer primary winding between an end of said conductor means adjacent the edge of one of said discs and said edge of said one of said discs, means connecting one end of said secondary winding to said one edge of said disc, a first radiating conductor connected between the other end of said secondary winding and the edge of the other one of said discs, a second transformer having a primary and secondary winding, means connecting the primary winding of said second transformer between the other end of said conductor means and the surface of said other one of said discs at said predetermined location, means connecting one end of the secondary winding of said second transformer to the surface of said other disc at said predetermined location, a second radiating conductor, said second radiating conductor being connected between the other end of the secondary winding of said second transformer and the surface of said one of said discs opposite said predetermined location.

7. In a wire grid lens antenna of the type wherein said two lens elements resemble two nesting discs and wherein said wire grid lens comprises conductor means extending from an edge of one of said lens elements radially inward a predetermined distance to the surface of the other of said lens elements, means for applying excitation to the end of said conductor means adjacent the edge of said one of said lens elements including first and second terminal means for connection to a source of energy, a capacitor connected between an end of said conductor means and said first terminal means, a transformer having a primary and a secondary winding, means connecting one end of said primary winding and said secondary winding and the edge of one of said grid elements to said second terminal means, a first resistor connecting the other end of said primary winding to said first terminal means, a dipole element having one end connected to an edge of said other of said lens elements, and means coupling the other end of said dipole element to the other end of said secondary winding including a second resistor and an inductance connected in series therewith, and constant resistance network means connecting the upper end of said conductor means to said surface of the other of said lens elements including a second dipole element connected between said constant resistance network means and the surface of said one of said lens elements at a location substantially opposite to the location to which said other end of said conductor means is coupled to said other of said lens elements.

8. The structure as recited in claim 7 wherein said constant resistance network means includes a transformer having a primary and a secondary winding, means connecting one end of said transformer primary and secondary windings to the surface of said other of said lens elements, an inductance connected between the other end of said secondary winding and one end of said dipole element means, a third resistor connecting the other end of said primary winding to the end of said conductor means at said predetermined location at the surface of said other of said lens elements, a capacitance, a fourth resistance connected in series with said capacitance, and means connecting said series connected capacitance and fourth resistance between said other end of said conductor means and said surface of said other one of said lens elements at said predetermined location.

9. The apparatus as recited in claim 7 wherein said conductor means comprises a first and second conductor, means spacing said first and second conductor apart for providing a constant characteristic impedance along the length thereof, and capacitive means coupling said spaced apart first and second conductors for eifectuating a loading thereof.

References Cited by the Examiner UNITED STATES PATENTS 2/1966 Tanner 343753 2/1966 Tanner 343753 

1. IN A WIRE GRID LENS ANTENNA OF THE TYPE WHEREIN SAID WIRE GRID LENS COMPRISES AN UPPER AND A LOWER LENS ELEMENT EACH BEING MADE OF WIRE GRID AND EACH HAVING SUBSTANTIALLY THE SHAPE OF A DISC, BOTH RESEMBLING TWO SPACED NESTING SHALLOW SAUCERS, SAID UPPER AND LOWER ELEMENTS BEING SPACED A PREDETERMINED DISTANCE FROM ONE ANOTHER, AN IMPROVED FEEDING STRUCTURE COMPRISING CONDUCTOR MEANS EXTENDING FROM AN EDGE OF ONE OF SAID TWO SPACED LENS ELEMENTS RADIALLY INWARDLY TO THE SURFACE OF THE OTHER OF SAID TWO LENS ELEMENTS, MEANS FOR APPLYING EXCITATION BETWEEN SAID EDGE AND SAID CONDUCTOR MEANS, AND MEANS FOR COUPLING SAID CONDUCTOR MEANS TO THE SURFACE OF SAID LENS ELEMENT TO WHICH IT EXTENDS. 